Multifrequency-current transmission



'Sept. 28, 1926. 1,601,074

B. W. KENDALL MULT lFREQUENCY CURRENT TRANSMISSION Filed-August 17. 1920 lnvemor'; Bur-fan Vl/. Kendall y M' /A'z Patentfcl Sept. 28,

[UNITED STATES PATENT OFFICE.

BURTON W. KENDALL. 0F YORK, N. Y.. ASSIGNOR TO WESTERN ELECTRIC COM-' 'PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK- MULTIFREQUENCY-CURRENT TRANSMISSION.

Application filed August systems particularly line conductors over which waves of di crent frequencies arev transmitted, it is well known that the waves of the higher'frequencies are less perfectly transmitted than those of the lower frequencies. A common instance of this is in telephone systems where the transm1tted currents comprise a considerable range of frequencies and the suppression of the hlgh voice fre uencies, due to the line characteristics, s ows itself in the impaired quality of the received voice waves. The wider the range of frequencies transmitted, the greater is the distortion due to the poorer transmission of the high frequencies relative tothe low.

' In carrier current systems, it is now common to transmit over thesameline currents of a range of frequencies many times as broad as the voice range. These frequencies may extend from a few hundred cycles such as the low frequencies of the voice, or even lower than any voice frequency, up to many thousand cycles. The currents of this wide range of frequency may represent many different messages, and it is necessary to select and separately receive these currents of different frequencies in order to carry on signaling. On lon transmission lines itis necessary or desire le to amplify all of these currents at one or perhaps several amplifying or receiving stations. It would be possible to separate t e currents of the frequencies used for the various messages, and to provide separate amplifying or repeating circuits for each of the several messages. This method would enable the higher frequency currents to be amplified toany desirable degree independently of the lower frequency currents and. thus correct in a measure for their greater attenuation. However, for a .considerable number of message channels,'the cost of the necessary separatin filters and of the large number of ampli ers would be excessive and it is highl desirable that one repeater amplify all 0 the currents for the same. reason that 17, 1920. Serial No. 404,180.

it is desirable to have one line transmit all of the currents.

In my copending application, Serial No. 334,681, filed October 31, 1919, Patent Numher 1,544,910, a multiplex repeater is shown which is capable of simultaneously amplifying a number of independent currents of different frequencies. If such a repeater is to be used for large range of frequencies on a line possessing unequal attenuating properties, it is desirable to associate with the repeater some means for equalizing or correcting or compensating for the distortion due to such attenuation. According to the present invention such progision is made in that repeating coils are used havmg a transmission efiiciency which increases in a predetermined manner with increasing frequencies.

In general, it will be desirable to introduce the means of correction for the line attenuation at the repeater since the correcting means introduces a loss which may be made up by the repeater. The presence of this correcting means also makes it possible to keep the'proper relation between the volumes of transmission of the various channels which, because of the characteristics of the line, will be equivalent to suppressing part of the lower frequency transmissions with respect to the hi her frequency transmissions. However, it will be understood that the invention is not necessarily limited to an arrangement in which the attenuation correcting device is combined with a repeater, but that this device may be applied to the line at any suitable point whether an amplifier is used or not. In the drawing to which reference will now be made for the purpose of givin a better understandin o the invention, ig. 1 is a'schematic out ay of a terminal station and a mid-line repeater in a system of the type in which the invention is ada ted to be used. Figs. 2 and 3 show repeating cir- .cuits embodying the invention or with which the invention is adapted to be associated. In Fig. 1 the main line ML is arranged for the transmission-of both low frequency currents such as those of the voice range and high frequency currents employed as carrier waves. The voice frequency currents are kept from passing'to the carrier apparatus at the extreme left of the figure by the interposition of the high pass ter HF which is so designed as to prevent the passage therethrough of currents of frequencies lower than the carrier frequencies, but to pass freely currents of all frequencies above the lowest frequency used in the carrier transmissions. The voice currents of the line ML are selectively transmitted through the low-pass filter LF in the line branch L leading to telephone sets or other low frequency signaling apparatus; Due to the attenuating properties of this filter for currents of high frequencies, the carrier frequency currents of the main lines ML are kept from the line L. The filters HF and LF may be of types disclosed in the patent to G. A. Campbell No. 1,227,113 granted May 22, 1917.

The carrier frequency branch contains a hybrid coil H and a balancing network so designed in connection with the hybrid coil as to enable currents from the transmitting loop TL to be impressed on the line without passing into the receiving loop RL. A number of carrier wave transmitting devices may be arranged for connection to the loop TL, two such sets being diagrammatically indicated. Such a transmitting set may comprise a microphone transmitter T, a modulator M of any suitable type for controlling the energy of the wave transmitted from the carrier frequency source G to the line, and a filter F, preferably a broad band filter, between the modulator and the loop TL. The generator G of another channel, it will be understood, produces a wave. of a different carrier frequency.

Carrier currents received over the line from a distant station pass into the receiving loop RL to which a number of receiving sets will ordinarily be connected. Each such current will be selected to the proper receiving channel by a filter, such as filter F, associated therewith, through which the respective currents selectively pass. A detector I) operating in. well-known manner enables the low frequency component to be heard in the receiver R.

The line ML it will be seen from the above description, serves to transmit currents of may be'used in either of the repeating branches ER 01' and which Wlll. now be described. V

Figure 2 represents a two-stage amplifier of the balanced type similar in general to that shown in my copending application above referred to. The thermionic amplifier tubes 1 and 2 are connected in balanced relation, sometimes termed a push-pull connection, and are arranged to feed into the second stage, composed of amplifiers 3 and 4:, similarly related to each other. By virtue of. the balanced connection, together with the proper proportioning of the potential sources 5, 6, 7 and 8, the amplifier circuit as a whole is made to operate with a substantially straight line characteristic, i. e. with the same amplifying factor for all of the impressed electromotive forces, and is therefore capable of repeating currents of a wide range of frequencies with a high degree of purity, and also currents of different frequencies and magnitudes since there is substantially no intermodulation of some frequencies by others. Potentiometers 9 and 10 aid in correctly balancing the circuit, and enable the voltage that is impressed on the amplifiers to be adjusted.

i ccording to the present invention the equalizing of the attenuation of the currents of the different frequencies is accomplished by means of transformer coils having different efficiencies at different frequencies. In Fig. 2 either the input transformer ll or the output transformer 12 or both may be coils having a gradually increasing efficiency with increasing frequency. The exact design of'transformer coils to secure this relation between efficiency and frequency varies with the different conditions to be met. However, it is well known to those versed in the art that the efiiciency of a transformer depends upon a number of factors, some at least of which depend upon the frequency. From known transformer formulae, it is possible to calculate the cf ficieucy at different frequencies as these factors are arbitrarily changed, and by controlling these factors it is possible to design a transformer to meet the required conditions.

The coils that are preferably used to practice the present invention have a low mutual impedance. It is well known that a transformer can be. represented by an equivalent network consisting of a line with a series impedance proportional to the leakage impedance of the primary coil, a second series impedance proportional to the leakage impedance of .the secondary coil and a shunt impedance proportional to the mutual impedance between the coils. In the present invention it is preferred to couple the coils closely so that the magnetic leakage is small. The Shunt impedance, which in the actual yond which increasing the frequency produces less increase and finally a decrease in the transformer efiiciency.

, By properly proportioning the mutual impedance relative to the line impedances it is possible to make the frequency range over which the loss due to the shunting effect decreases with increasing frequency, coincide with that frequency. range over which compensation is desired.

In the systems illustrated, to which the invention is applicable, the coils should have a relatively low mutual impedance in order to bring the correcting action within the range of the frequencies transmitted. This low mutual impedance may be secured in a number of possible ways, and in general by using a transformer which has small inductance in the primary and secondary windings. This indicates, for any given transformer ratio which the given conditions may require, a relatively small number of turns in both rimary and secondary coils. The permeabi ity of the core and the reluctance of the magnetic circuit also influence the mutual impedance as is well known. a

It may be stated that in the transformers used experimentally in the development of the present invention, those losses such as eddy current and hysteresis losses, which increase in frequency, were reduced to small value by the use of so-called dust cores similar in general to those disclosed in United States patent to Speed No. 1,274,952, granted-August 6, 1918. p

In Fig. 3 the input coil 13 accomplishes part of the compensation, and another part of the correcting-is accomplished by means of the output transformer 14. This use of a plurality of transformers of course permits a greater flexibility in the compensating arrangement than would be practicable with a single transformer. An additional transformer 15, shown in Fig. 3, is inserted to diminish the possible effects of the capacity between the transformer windings. In Fig. 3 but one stage of amplification is'indicated, and this is all that will benecessary in some instances. However, a. greater number of stages ma be used as desired. 7

By the use of t e present invention,.it has been found possible to correct for the line attenuation over a relatively large frequency range. This makes practicable the use of asingle repeater for a multiplex or any other line over which a range of frequen- 'cies is transmitted. his to be understood that the means for carrying out the invention will be varied considerably to suit conditions, and that the invention is not therefore to be construed as limited to the articular arrangements that have been ilustrated and described. but only by the scope of the appended claims.

What is claimed is: v

1. In combination, a line, means for transmitting thereover currents of a range of frequencies including aplurality of carrier waves for multiplex telephony, the attenuation of which line varies with the frequency of the transmitted currents, and a transformer associated with said line having a continuously difi'ering transmission efficiency throughout said range of frequencies for correcting for the unequal line attenuation of said transmitted currents by transmitting with greater efliciency the currents that are the more highly attenuated by said line.

2..In a multiplex telephone system, a line, the attenuation of which varies with the frequency of the currents transmitted thereover, means for transmitting simulv taneously thereover a plurality of speech modulated carrier waves, a repeater in said line for repeating currents of all the freuencies' on said line substantially uniormally, and a transformer for transmitting between the line and the repeater, currents of all the frequencies on said line, said transformer having such a gradually differing transmission eiiiciency throughout the frequency range of said currents as substantially to compensate for the unequal attenuation by said line of the currents of the different frequencies.

3. In a multiplex signaling system, a line, the attenuation of which varies with the frequency of the transmitted currents, means for simultaneously transmitting thereover a plurality of carrier waves of different frequencies occupying a frequency range several times as extensive as that embraced by essential speech frequencies, a single amplifying repeater circuit for substantially uniformly repeating all of the transmitted currents, and a transformer associated with said repeater and line having a gradually increasing transmission efiiciency with increasing frequency throughout said range of transmitted currents for compensating for the unequal attenuation of said currents by said line.

4. In a signaling system, a-line, means for transmitting thereover audible frequency currents representing a message together with inaudible frequency currents representing a diflerent message, the attenuation of said line varying with the frequency of the transmitted currents and a repeating coil associated with said line having a transmission loss which varies with the frequency of the currents throughout the totalfrequency range of the currents transmitted over said line in a manner substantially complemental to the variation with frequency of said line attenuation whereby the distortion due to variable line attenuation is substantially corrected for.

5. In a multiplex telephone system, a line for transmitting simultaneously currents representing a plurality of carrier telephone messages, reenforcing repeater means for all of said currents and a common means for impressing all of said currents simultaneously on said repeater means and for correcting for the line distortion for all of said currents.

6. In a multiplex signaling system, a line divided into sections, said line as a whole, having increasing attenuation with increasing frequency, means to transmit over said line a plurality of carrier waves of distinct frequencies each controlled by signals, said signal controlled carrier waves occupy ng a frequency range several times as extens ve as the range'of essential speech frequencies, and means for compensating for the unqual line attenuation of the different frequencies comprising a transformer connected between adjacent line sections for transmitting all of said signal controlled Waves, said transformer having a lowmutual impedance and a transmission efficiency increasing with increasing frequency from the lowest to the highest frequencies in the range transmitted over said line in substantially the same manner as the attenuation of said line increases with frequency.

7. In a carrier telephone system, a line divided into sections, said line having a gradually changing attenuation with frequency throughout the total frequency range of currents transmitted over it, means to transmit over-said line a plurality of speech modulated carrier waves, and a single repeater circuit between adjacent line sections for amplifying all of the transmitted currents, said repeater circuit including a means serving in common for all of said carrier waves for causing the modulated carrier waves of the higher frequencies received over one line section to be passed to the next line section with greater amplification than the modulated carrier waves of lower frequencies, to compensate for the unequal line attenuation of the currents of the different frequencies.

In witness whereof, I hereunto subscribe my name this 11th day of August A. D., 1920.

BURTON W. KENDALL. 

